Antenna arrays are well known, and are finding increasing use. Active antenna arrays which switch from transmit to receive modes, for use in systems such as radar, include both power amplifiers and low-noise receivers, and some method for switching from transmit mode to receive mode. This mode of operation is advantageous in that it allows the antenna elements themselves, and possibly the control elements, including the phase shifters and the attenuators (or amplifier gains), to be used for both the transmit and receive modes of operation.
In some applications, such as repeaters, the signals which must be handled are continuously received, and must be continuously retransmitted. For repeater use, active antenna arrays which switch from transmit to receive operation are not useful, for they must give up one of transmission or reception while performing the other function.
In critical applications, there must be some way to align the antenna beam(s) of an array antenna. The alignment is performed by controlling the phase shifters and or attenuators associated with each antenna element or group of antenna elements in such a manner as to generate the desired beam shape and direction. Antenna controllers, also known as Antenna Control Units (ACU) which provide such control are also well known. In digitally controlled systems, the phase shifters and the attenuators (which may include gain control of an amplifier) are controlled by digital signals. The smallest unit of control which can be achieved in a digital system is defined by a one-bit change in the signal. The phase change provided by a phase shifter, and the attenuation change provided by an attenuator, are controlled by a multibit control signal, as for example a five-bit control signal, in which any one value represents one of 32 possible states. When the number of bits of the control signal is so limited, the corresponding change in control provided by the phase shifter or attenuator is usually the maximum available change divided by the number of states represented by the control signal. In the five-bit control signal example, assuming that the maximum possible phase shift provided by a phase shifter is 360.degree., the smallest increment of control is designed to be 360.degree. divided by 32, or slightly more than 10.degree. per bit.
The actual phase shift of a phase shifter, and the actual attenuation of an attenuator, at a given value of the digital control signal, may deviate from the nominal value. The cumulation of these errors may substantially affect the accuracy with which the ACU can point the beam(s) in the desired direction, and or establish the desired beamshape. For this reason, various calibration schemes have been proposed. In this context, the term "calibration" means the process of determining the (one-to-one) relationship between the phase or amplitude of the input and output signals of a controllable phase shifter or attenuator for a given control input signal state. One simple calibration scheme is to measure the phase shift of each phase shifter, and the attenuation of each attenuator, before it is mounted in the antenna array, and to provide the resulting data to the ACU as an indication of the expected phase or attenuation of the control unit in the presence of a given digital input signal. This type of calibration scheme does not take into account changes which may occur in the performance of the various control elements due to aging, voltage variations which may be experienced, temperature effects, transmission-line impedance effects, and the like.
Improved calibration arrangements are desired.